The standard theory says ego depletion — a tired brain has fewer resources left to exert self-control or make good decisions. This helps explain why google goes out of their way to make sure their employees make fewer decisions — so they can put more energy and effort into the more important ones.
“A bad mood is no different. When we push our mind too hard, asking it to refrain from carbs and cigarettes, we struggle to avoid the negative thoughts and emotions that lead to sour moods,” says Jonah Lehrer:
In a series of clever studies, the Northwestern psychologists David Gal and Wendy Liu demonstrate that the exertion of self-control doesn’t just make it harder for us to contain our own anger – it also make us more interested in watching anger-themed movies, or thinking about anger-related information, or looking an angry facial expressions. In other words, acts of self-control haven’t just exhausted the ego – they actually seem to have pissed it off.
My favorite experiment involved movies. Two hundred and thirty nine subjects were given a choice between a virtuous apple and a hedonistic chocolate bar. (A slim majority chose the apple.) Then, they were offered a selection of movies to watch, from Anger Management (an anger themed film) to Billy Madison (a non-anger themed film.) Interestingly, students were significantly more likely to choose the angry films if they’d first chosen the apple. And it wasn’t just films: another experiment found that people who exercised financial restraint – they chose a gift certificate for groceries over one for spa services – were more interested in looking at angry faces.
What’s driving this effect? Gal and Liu argue that the preference for angry stuff is not simply a result of ego depletion. Instead, they speculate that self-control is inherently aggravating. Perhaps choosing the apple annoys us because our goals have been thwarted – we really wanted the candy bar – or maybe we’re pissed because we feel that our sense of autonomy has been diminished. (If we weren’t so constrained by societal norms and expectations, we would have gorged on chocolate.) The point is that the labor of self-control directly inspires our tendency towards anger, and not indirectly via a worn down prefrontal cortex.
Still curious? Read Baumeister's book Willpower: Rediscovering the Greatest Human Strength
The next time you resist that chocolate bar you should keep a few cognitive resources in reserve (and have a Coca Cola).